Chassis are frameworks used to house telecommunications equipment, and are often found in a room or other area where, for example, switching takes place. This other room or area is often referred to as a "central office environment." When stacking chassis in a central office environment, important considerations include, among others, the proper cooling of each chassis, the efficient stacking of chassis to avoid wasted space, and meeting certain environmental requirements. The installer of telecommunication equipment has to be certain that lower chassis do not preheat upper chassis or that upper chassis do not block air flow from lower chassis. This was usually solved by perforating the tops and bottoms of chassis to obtain good vertical cooling. However, new network equipment-building standards ("NEBS") by Bell Atlantic and other RBOC companies discourage the design of complete vertical cooling because fire in lower chassis can easily pass through to upper chassis. Therefore, most chassis are now designed with substantially solid tops, and chassis are stacked in a rack with gaps or vent hoods between them. Usually, the gaps between chassis are 1.75"-3.5" (1.75" is referred to herein as one "rack unit"). And since space is at a premium when stacking and arranging telecommunication equipment, this wastes valuable space.